I tried to get more information from Sitecore developer reference documents, but I could not find much about where and how the "Invoice Unit Price" property on inventory is used.
Optionally, you can change the default Invoice Price Currency value, and enter the Invoice Unit Price, Invoice Price Currency, and whether the item is available for preorders or backorders, the dates it is available, and the limits for preorders and backorders.
Also there is a note:
Preorder and backorder functionality is not fully implemented in this release (10.1).
So looks like those properties were introduced early then functionality was implemented.